Getting Futaba's DG channels (digital only) to work with a PB Mercury

I have 13 functions on the plane and assigned one of them (a Jet-tronics valve opening the canopy) to the DG1 channel (which, I assumed, could be channel 13, but it does not seem to work that way). It works when using a 6014HS receiver and plugging the valve on the DG1 output. However, the Mercury does not seem to recognise the DG channel as channel 13, when I map it in the box' output menu. All 12 other functions are working fine except for this one .

Question: how do I get the digital channel to be recognised by the box?